The Global Positioning System (GPS) consists of a constellation of globally-dispersed satellites with synchronized atomic clocks that transmit radio signals. Time, as maintained by each satellite, is embedded in the transmitted radio signal of each satellite. The difference between the time embedded in a satellite's radio signal and a time measured at the point of reception of the radio signal by a clock synchronized to the satellite clocks is a measure of the range of the satellite from the point of reception. Since the clocks in the system cannot be maintained in perfect synchronism, the measure of range is referred to as "pseudorange" because it includes both a satellite clock error and the clock error at the point of reception.
Each satellite transmits, in addition to its clock time, its position in an earth-fixed coordinate system and its own clock error. A user, by measuring the pseudoranges to four satellites and correcting the pseudoranges for the satellite clock errors, can first of all determine his actual range to each satellite and his own clock error. The user can then determine his own position in the earth-fixed coordinate system, knowing his range to each of the four satellites and the position of each satellite in the earth-fixed coordinate system.
The GPS satellites may be an unsatisfactory source of navigation information. A slowly increasing range bias error can occur due to satellite clock faults or due to errors in the uploaded data, which may not be detected or corrected for several hours. The Federal Aviation Authority (FAA) requires that, even for approval as a supplemental navigation system, the system have "integrity" which is defined by the Federal Radio Navigation Plan (U.S. Dept. of Defense, DOD-4650.4 and U.S. Dept. of Transportation, DOT-TSC-RSPA-87-3 1986, DOT-TSC-RSPA-88-4 1988) as the ability to provide timely warnings to users when the system should not be used for navigation. For sole means of navigation, the system must also have sufficient redundancy that it can continue to function despite failure of a single component. Because navigational integrity is a critical component of an aircraft flying in civil airspace, methods have been developed for assuring that minimum integrity standards are met. Currently, there are three types of solutions available to carry out this function, each achieving different degrees of success.
(1) "Receiver autonomous integrity monitoring" (RAIM) is known wherein a receiver makes use of redundant satellite information to check the integrity of the navigation solution. With the present GPS constellation, there are insufficient satellites to provide worldwide RAIM coverage unless other satellite navigation sources or altimetry are included. To detect a satellite failure using RAIM requires that at least five satellites with sufficiently good geometry be available. For a sole means of navigation, it is also necessary to isolate erroneous satellites and to be able to navigate with the remaining satellites. Given the predetermined level of integrity and acceptable level of false alarm, the usefulness or shortcomings of RAIM is assessed by its availability to the user for a given protection limit dictated by the phase of flight. The availability numbers, which have been widely researched and published for the operational GPS satellite constellation and the visibility geometries it provides, are less than sufficient for sole means in oceanic operation and even worse for enroute, terminal and non-precision approach operations.
(2) Another known solution utilizes GPS satellites augmented with a high-quality Inertial Reference System (IRS). One such example is the Litton AIME solution described in U.S. Pat. No. 5,583,774 to Diesel entitled ASSURED-INTEGRITY MONITORED-EXTRAPOLATION NAVIGATION APPARATUS, which is herein incorporated by reference. A high-quality IRS utilizes the inertial sensors of a high quality inertial measurement unit (IMU) to maintain an inertial navigation solution When augmented by additional information from a high quality IRS, the availability of RAIM improves considerably to the point where the above limitations are negated, except for precision approach. However, this method relies heavily on the high stability characteristics of the IRS. To effectively augment the RAIM, the IRS must be independent of the GPS (unaided by GPS) over a rather long duration of time (20-30 minutes). This requires a very high quality IMU (with a gyro drift of less than 0.01 degrees per hour), since an unaided tactical-grade inertial measurement unit (with a gyro drift rate of 1-10 degrees per hour) solution becomes unusable in minutes.
(3) The FAA's Wide Area Augmentation System (WAAS) is another potential solution for providing integrity via ground monitoring by an elaborate network of stations and communications via geostationary satellites. Similar systems are under development in Europe (EGNOS) and Japan (MTSAT). However, all these solutions are subject to coverage limitations. In the Oceanic or Arctic phases of flight, this problem is even more significant and apparent because the lack of any other navigation.
(4) Alternate satellite navigation systems are under development which are comparable to GPS capability as stand alone systems but can be used in conjunction with GPS to provide augmented lines of position sources. These include, the Russian GLONASS (with a partial constellation in orbit) and the proposed European Galileo (which is under design). They are designed to be standalone systems with integrity limitations similar to GPS.